1. Field of the Invention
The present invention relates to, for example, a transistor, a semiconductor device, and manufacturing methods thereof. The present invention relates to, for example, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, a processor, and an electronic device. The present invention relates to a method for manufacturing a display device, a liquid crystal display device, a light-emitting device, a memory device, and an electronic device. The present invention relates to a driving method of a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a memory device, and an electronic device.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter.
In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A display device, a light-emitting device, a lighting device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
Electro-optical devices including large-sized integrated circuits formed using transistors in which crystalline semiconductor films (typically, a polysilicon film, a microcrystalline silicon film, and the like) provided over insulating surfaces are used as active layers have been actively developed. Transistors including polysilicon films can form various functional circuits because of its high field-effect mobility.
For example, in an active matrix liquid crystal display device, a pixel circuit for performing image display for each functional block and an integrated circuit including a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and the like, which are based on a CMOS circuit, are formed over one substrate.
In these circuits, a voltage is applied to liquid crystals to drive a pixel portion including a pixel transistor, which functions as a switching element, and a capacitor. Since the liquid crystals need to be driven with an alternating current, a method called frame inversion driving has been employed in many cases. Thus, the transistor characteristic of a sufficiently low off-state current (Ioff: the value of a drain current flowing when a transistor is off) is required. However, it has been pointed out that a transistor including a polysilicon film has disadvantages of a low drain withstand voltage and a large off-state current.
As a means for solving this problem, a lightly doped drain (LDD) structure including a low-concentration impurity region (LDD region) (a structure where the low-concentration impurity region is provided between a channel formation region and a source region or a drain region to which an impurity element is added at a high concentration) has been known.
In the case where the LDD structure is employed, as compared with the case where a single-drain structure is employed, the drain withstand voltage is significantly improved; however, the drain current is reduced because of the large resistance of the low-concentration impurity region (an n− region or a p− region).
In addition, it has been pointed out that there is a problem in that a high electric field is generated in the vicinity of the drain region, hot carriers are generated and trapped in a gate insulating film over an LDD region, and element characteristics such as a threshold voltage are largely changed and decreased. To prevent the degradation due to hot carriers, the transistor where the LDD region and the gate electrode overlap has been disclosed (for example, see Patent Document 1). As compared with a transistor having a normal LDD structure, a transistor having a gate-overlap LDD structure has high current drive capability and can relieve the high electric field in the vicinity of the drain region to prevent the degradation due to hot carriers.
Furthermore, since a high drive voltage is applied to a buffer circuit, the withstand voltage needs to be increased such that the buffer circuit is not broken even if a high voltage is applied, and it is also necessary to secure a sufficiently large on-state current value (Ion: the value of a drain current flowing when a transistor is on) to obtain high current drive capacity.
As a means for achieving high-speed operation of a transistor, for example, a dual gate structure where a semiconductor layer is sandwiched between a pair of gate electrodes, which is disclosed in Patent Document 2, has been considered. In addition, a fin structure disclosed in Patent Document 3 has been proposed.
Moreover, to reduce variations in luminance of an OLED, a structure where a transistor having a long channel length is used is disclosed in Patent Document 4.